[gcc r13-3388] c++: Mitigate -Wuseless-cast with classes [PR85043]

[Marek Polacek <mpolacek@gcc.gnu.org>]

https://gcc.gnu.org/g:b3c98d6a59a6dcd5b0b52bd5676b586ef4fe785f

commit r13-3388-gb3c98d6a59a6dcd5b0b52bd5676b586ef4fe785f
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Oct 18 12:20:14 2022 -0400

    c++: Mitigate -Wuseless-cast with classes [PR85043]
    
    -Wuseless-cast (not part of -Wall/-Wextra) warns here:
    
      struct S { };
      void g (S&&);
      void f (S&& arg)
      {
        g (S(arg)); // warning: useless cast to type 'struct S'
      }
    
    which is wrong: the code will not compile without the cast because
    "arg" is an lvalue which cannot bind to S&&.
    
    This patch disables the warning when an object that isn't a prvalue
    is cast to a non-reference type.  Therefore we still warn about the
    useless cast in "X(X{})".
    
            PR c++/85043
    
    gcc/cp/ChangeLog:
    
            * typeck.cc (maybe_warn_about_useless_cast): Don't warn when
            a glvalue is cast to a non-reference type.
    
    gcc/ChangeLog:
    
            * doc/invoke.texi: Update documentation of -Wuseless-cast.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/warn/Wuseless-cast.C: Remove dg-warning.
            * g++.dg/warn/Wuseless-cast3.C: New test.

Diff:
---
 gcc/cp/typeck.cc                           | 12 +++++++-----
 gcc/doc/invoke.texi                        | 13 ++++++++++++-
 gcc/testsuite/g++.dg/warn/Wuseless-cast.C  | 12 ++++++------
 gcc/testsuite/g++.dg/warn/Wuseless-cast3.C | 29 +++++++++++++++++++++++++++++
 4 files changed, 54 insertions(+), 12 deletions(-)

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index da0e1427b97..16e7d85793d 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -8104,11 +8104,13 @@ maybe_warn_about_useless_cast (location_t loc, tree type, tree expr,
   if (warn_useless_cast
       && complain & tf_warning)
     {
-      if ((TYPE_REF_P (type)
-	   && (TYPE_REF_IS_RVALUE (type)
-	       ? xvalue_p (expr) : lvalue_p (expr))
-	   && same_type_p (TREE_TYPE (expr), TREE_TYPE (type)))
-	  || same_type_p (TREE_TYPE (expr), type))
+      if (TYPE_REF_P (type)
+	  ? ((TYPE_REF_IS_RVALUE (type)
+	      ? xvalue_p (expr) : lvalue_p (expr))
+	     && same_type_p (TREE_TYPE (expr), TREE_TYPE (type)))
+	  /* Don't warn when converting a class object to a non-reference type,
+	     because that's a common way to create a temporary.  */
+	  : (!glvalue_p (expr) && same_type_p (TREE_TYPE (expr), type)))
 	warning_at (loc, OPT_Wuseless_cast,
 		    "useless cast to type %q#T", type);
     }
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index c176e2dc646..cd4d3c1d72c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -4551,7 +4551,18 @@ pointers after reallocation.
 @item -Wuseless-cast @r{(C++ and Objective-C++ only)}
 @opindex Wuseless-cast
 @opindex Wno-useless-cast
-Warn when an expression is casted to its own type.
+Warn when an expression is cast to its own type.  This warning does not
+occur when a class object is converted to a non-reference type as that
+is a way to create a temporary:
+
+@smallexample
+struct S @{ @};
+void g (S&&);
+void f (S&& arg)
+@{
+  g (S(arg)); // make arg prvalue so that it can bind to S&&
+@}
+@end smallexample
 
 @item -Wno-conversion-null @r{(C++ and Objective-C++ only)}
 @opindex Wconversion-null
diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
index 2fd6bc45102..d7cb89930a6 100644
--- a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
+++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C
@@ -62,11 +62,11 @@ A prvalue();
 
 void f()
 {
-  int n; 
+  int n;
 
-  (int)(n);                    // { dg-warning "3:useless cast" }
-  static_cast<int>(n);         // { dg-warning "3:useless cast" }
-  reinterpret_cast<int>(n);    // { dg-warning "3:useless cast" }
+  (int)(n);
+  static_cast<int>(n);
+  reinterpret_cast<int>(n);
 
   (int*)(&n);                  // { dg-warning "3:useless cast" }
   const_cast<int*>(&n);        // { dg-warning "3:useless cast" }
@@ -100,8 +100,8 @@ void f()
 
   A a;
 
-  (A)(a);                     // { dg-warning "3:useless cast" }
-  static_cast<A>(a);          // { dg-warning "3:useless cast" }
+  (A)(a);
+  static_cast<A>(a);
 
   (A*)(&a);                   // { dg-warning "3:useless cast" }
   const_cast<A*>(&a);         // { dg-warning "3:useless cast" }
diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C
new file mode 100644
index 00000000000..43dd2ebfaea
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C
@@ -0,0 +1,29 @@
+// PR c++/85043
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wuseless-cast" }
+
+struct S { int s; void bump () { s++; } };
+
+void
+foo ()
+{
+  S s = { 1 };
+  s.bump ();
+  S (s).bump (); // { dg-bogus "useless" }
+  ((S) s).bump (); // { dg-bogus "useless" }
+  static_cast<S> (s).bump (); // { dg-bogus "useless" }
+}
+
+struct X { };
+void g(X&&);
+
+void
+f (X&& arg)
+{
+  g(X(arg)); // { dg-bogus "useless" }
+  g(X(X{})); // { dg-warning "useless" }
+  g(static_cast<X&&>(arg));
+
+  int i = (int) 1; // { dg-warning "useless" }
+  const int &r = (int) i; // { dg-bogus "useless" }
+}